[S01-P] Poster

[S01-P-20] Distribution of deep earthquakes in the subducting Pacific slab beneath Japan

^○Ayako Tsuchiyama¹, Junichi Nakajima¹, Toru Matsuzawa² (1.Tokyo Institute of Technology, Tokyo, Japan, 2.Tohoku University, Miyagi, Japan)

Generally, the number of earthquakes in slabs decreases with depth. However, the seismicity increases again in a depth range of 400–500 km. The mechanism of this phenomenon remains poorly understood, even though many hypotheses for the genesis of deep earthquakes have been proposed, which include dehydration-embrittlement hypothesis, transformational faulting, and shear instability. In this study, we analyzed 93 deep earthquakes (M≥3.0) that occurred at depths of >300 km beneath Tokai area in Japan to understand factors that control the high seismic activity in the mantle transition zone.
First, we read P-wave polarities and determined focal mechanism solutions of earthquakes. Then, we picked arrival times of both P and S waves and relocated hypocenters by using double-difference earthquake relocation algorithm (Waldhauser and Ellsworth, 2000). The relocated hypocenters do not show a double-planed seismicity as is observed in Iidaka and Furukawa (1994). In the next step, we will relocate hypocenters with differential travel-time data derived from waveform cross correlations and constrain hypocenter locations with high accuracy. We will present detailed hypocenter distributions together with focal mechanism solutions, and discuss a plausible mechanism that facilitates deep earthquakes in the mantle transition zone.